1. Field of the Invention
The present invention includes ultra low carbon steel compositions. More particularly, the ultra low carbon steel compositions are weathering steels having ultra low carbon compositions with copper, chromium, nickel, titanium, niobium and boron components. Most particularly, the steel of the present invention exhibits good weldability and low temperature fracture toughness at a minimum yield strength of 65,000 psi in plate sections of two inch thickness. The ultra low carbon compositions are useful in structural steel applications, such as ship and highway bridge construction.
2. Brief Description of the Related Art
Ultra low carbon bainitic (ULCB) steels rely on extremely low carbon contents and high hardenability to obtain good combinations of strength and toughness due to the fine bainitic ferrite that forms and the limited, and possibly nonexistent, amount of carbides. High strength and toughness are obtained in a ULCB steel with proper plate processing. Typical ULCB steels have high levels of nickel and molybdenum to obtain the required hardenability while other ULCB steels rely on boron microalloying.
Weathering steels are known, as are the advantages inherent in their use. Weathering steels develop a thin film of oxide that protects the underlying metals from further oxidation. As such, the weathering steel forms its own protective coating and does not require a separate covering. Paint protection against corrosion is not required of the weathering steels, allowing for ease of maintenance once incorporated into a structure. The thickness of the metal oxide layer generally is less than 0.1 mm, requiring between two and four years to form and stabilize. Weathering steels are economically desirable despite the higher cost relative to plain carbon steel. Weathering steels possess several drawbacks in addition to initial cost. After two or four years, the final appearance of the exposed surface may look rusty or otherwise unattractive. Additionally, the toughness of weathering steels is generally not dependably reproducible, limiting their use.
There is a need in the art to provide ultra low carbon weathering steel having good weldability and low temperature fracture toughness for structural steel applications. The present invention addresses this and other needs.